1. Field of the Invention
The present invention relates to a test pattern printing method, an information processing apparatus and a printing apparatus, and more specifically to a test pattern printing method for printing a test pattern which is checked to detect a color deviation, the deviation being caused by variations in an output characteristic among print heads, and an information processing apparatus and a printing apparatus for performing printing of the test pattern.
2. Description of the Related Art
As devices for automated office work have come into wide use, opportunities for producing color images in offices have increased. Known output devices for outputting color images include printing apparatuses of various methods, such as an ink jet method, an electro-photographic method and a heat transfer method. These printing apparatuses have been able to output picture-like color images of relatively high quality in recent years.
Such printing apparatuses generally print color images using three colorants of chromatic colors, cyan (C), magenta (M) and yellow (Y), or four colorants with an additional achromatic color, black (K). The printed color images, however, may have a color deviation or color misrepresentation (e.g., produced color in the printed image differs from a desired one) due to an imbalance of output characteristics (reflection density or lightness, saturation, hue, etc.) among print heads for respective colors. In the ink jet system, for example, the print heads may have, among themselves, differences in an amount of heat produced by heaters (film thickness of the heater) for ejecting ink and differences in a size and shape of ink ejection orifices, causing variations in an amount of ejected ink. This in turn may result in the output characteristic imbalance among the print heads for individual colors, as described above. Not only do the print heads inherently have such differences, but it is also known that these differences are produced over time.
To cope with this problem, a personal computer functioning as a host computer for the printing apparatus, or the printing apparatus itself, may perform correction processing which involves detecting the color deviation due to the differences among respective print heads and, based on the result of detection, correcting the output characteristics of the print heads to have predetermined characteristics. Known correction processings include methods to be carried out in two ways, which are distinguished depending primarily on whether the detection of the color deviation is done by using an input device such as a scanner or by a human visual check. These two correction methods, as distinguished by the manner of detecting the color deviation, will be briefly explained below.
The method using the input device such as a scanner, for example, firstly prints a patch pattern by using the print heads of C, M, Y and K coloring materials that are the subjects of the correction, as disclosed in the official gazette of Japanese Patent No. 2661917. Then, the patch pattern is read by the scanner to detect a difference between a read value (representing an output characteristic of the head) and an expected value calculated from the print data for the patch pattern. Based on the detected difference, the color deviation is corrected. The patch pattern used in this method may be a solid pattern of each of C, M, Y, K colors or a patch pattern of each color having patches showing a gradation in density. The gradation patch pattern can also be used to detect an output characteristic with respect to a halftone, thus improving precision of detecting the color deviation. A method is also known as that combines the C, M, Y and K colors to form second- or third-color patch patterns and thereby improves the detection and correction accuracy.
On the other hand, the method using the visual check, unlike the method using the input device, cannot easily detect an absolute value with respect to the output characteristic for each color head. Hence, a detection method that uses a third-color patch pattern printed by mixing three colorants C, M, Y is mostly used. More specifically, a plurality of patches of almost gray color is printed. The plurality of patches include a patch as a central patch of the plurality of patches, which is expected to be printed at a predetermined ratio of three colorants (represented by print data of three colorants) so as to be a patch of an achromatic color when the patch is printed by using print heads with an average ink ejection amount or without any deviation of ink ejection amount, and other patches which are printed with their three-color ratios successively changed slightly. From the plurality of patches, a user visually chooses one patch closest to the achromatic color so as to detect the patch of the most appropriate output characteristic balance among the C, M, Y color print heads. Then, the correction data corresponding to the detected patch is used to correct the output characteristic of each print head. This method of detecting the color deviation takes advantage of a fact that a slight imbalance in the output characteristics among the C, M, Y color print heads causes influence of the color having relatively large output characteristic change to appear in the patch, resulting in the patch deviating from the achromatic color.
While the color deviation and the method of its detection have been described for the case of the print heads of the ink jet system, differences in the output characteristics of the color print heads may also occur with the printing apparatuses of the electrophotographic system and the heat transfer system due to causes peculiar to the principles of the respective printing systems. In these printing systems, also, the color deviation detection and correction are performed in a similar manner.
The conventional color deviation detection method described above, however, has the following drawbacks.
First, in a usual environment in which the printing apparatus is used, the method using an input device such as a scanner requires a condition that the user has an input device. Not all users have an input device available and thus the method using the scanner or other input devices is not realistic. If some input devices are made available, the correction process on a basis of the color deviation detection using such wide variation of input devices is often very difficult to be carried out.
The method based on the visual check, on the other hand, does not require any special input device and thus can be employed by any user to detect the color deviation. It is, however, not so easy to select a patch closest to an achromatic color from a plurality of patches with their C, M, Y color ratios progressively changed slightly.
For example, JIS (JIS E3305, JIS Z8721, JIS L0600, etc.) and various other organizations provide specifications concerning a color difference (xcex94E). In these specifications, a range of the color difference of 3.2-6.5 is defined as a xe2x80x9crange that can be handled as the same color in terms of impression.xe2x80x9d This suggests that visually picking up a patch closest to the achromatic color from patches in this range of color difference is difficult. It is also stated that the color difference in such a range may xe2x80x9ccause a customer to complain about difference in color when selecting paint color,xe2x80x9d suggesting that even in this range of color difference, if a wrong patch is selected, an image printed after being corrected by a correction value corresponding to the selected patch may fail to provide a desired color.
For this reason, an effort is being made to improve an accuracy of the visual check-based color deviation detection method.
FIG. 1 is a view schematically showing an example of a test pattern that enables an improvement of precision for detecting the color deviation. In FIG. 1, each frame represents a patch printed with a mixture of C, M and Y colors and with K. Four numbers in each of two areas in each patch represent multi-valued gradation data for printing the associated area and correspond, from top to bottom, to C, M, Y and K. Thus, the patch shown in FIG. 1 consists of two upper and lower areas. The test pattern is made up of an array of such patches with their gradation values of M increasing vertically downward in five steps and that of C increasing horizontally toward the right in five steps.
As is apparent from FIG. 1, a test area defined by the upper area in each patch is an area printed with a mixture of C, M, Y colorants of chromatic colors (the mixed color is hereinafter referred to as xe2x80x9cPCBkxe2x80x9d that is xe2x80x9cprocess blackxe2x80x9d). A reference area defined by the lower area is an area printed with only K, a colorant of achromatic color. In the upper area printed with PCBk the gradation value of Y is fixed at a 128 level. Further, as described above, value of C in the pattern increases toward the right and value of M increases downwardly.
With the test pattern constructed in this way, the precision of visual detection can be improved. More specifically, this pattern takes advantage of a visual characteristic that when objects are close together, a small color difference can be perceived. More concretely, this pattern is based on a fact that even if the color difference is as small as 0.8-1.6, a comparison between adjacent objects allows a human eye to perceive the color difference. The pattern of this method allows the user, rather than to search, without any criterion, through a plurality of patches printed with only PCBk to find a patch closest to the achromatic color, to make a one-to-one comparison between an area printed with an achromatic color K and an area printed with PCBk, which areas are adjacent to each other, and thereby select a patch with a PCBk area having the least color difference from the area of achromatic color K. Thus, the precision of visual detection can be improved. This method is hereinafter called an xe2x80x9cadjacency comparison methodxe2x80x9d.
As described above, the precision of visual detection of color difference (color deviation) can generally be improved by using the adjacency comparison method. However, when it is attempted to make a further improvement in the detection precision, particularly in the two-value or binary value printing, the conventional adjacency comparison method may prove insufficient. This problem will be explained in detail as follows.
The test area of PCBk in the conventional patch is generally printed not by what is called a solid printing in which dots of each colorant are formed on all pixels, but by half-tone printing using an intermediate gradation value of each colorant. The half-tone printing is used for the following reason. Considering that there are errors in the positions of the dots formed and that the dots are circular in actual printing, a printing system is designed to overlap the formed dots to ensure that during the solid printing the paper surface is filled with dots so that no blank areas are left unprinted. That is, because the printing system is designed as described above, when a printing duty is near 100%, a rate of covering the paper surface with the dots is about 100% to hardly change, which in turn does not allow the variation in a size of the formed dot and a position of the formed dot due to a change of the output characteristic to be detected as a significant change. Therefore, the PCBk area is not solid-printed for each colorant dot, but is often printed with gradation data that corresponds to an intermediate value of the printing duty. More specifically, of 256 gradation values represented as data of 8 bits, the data close to a value of 128 is used for printing the PCBk area. As for an adjacent area printed with the K colorant, it is desired that a difference in lightness of the adjacent area from the above-described PCBk area is not so large. Thus, the data near the gradation value of 128 is also often employed in this case.
With this conventional patch, however, when the test area printed with chromatic colorants at the gradation value of 128 and a reference area printed with only K colorant at the gradation value of 128 are compared, there is a relatively great difference in a dot spatial frequency (dot density) between the areas, particularly in the case of printing the patch with a printing apparatus of the binary printing system. As a result of this, respective characteristics of visual perception for the two areas under comparison differ greatly, rendering proper detection of the color deviation by the visual check impossible.
In the test area, even though the C, M, Y colors each have the gradation value of 128, the total number of dots formed per unit area is roughly three times the number of dots in the reference area printed with only K at the gradation value of 128. On the other hand, in order to make the spatial frequency of the test area almost equal to that of the reference area, the number of dots of each C, M, Y color may be set to be equal to one-third the number of dots in the reference area, i.e., the gradation value of each color is set to about 42. In this case, however, a large difference in the lightness is generated between the two areas, rendering the patch inappropriate for comparison.
That is, for the printing apparatus of the binary printing system, attempting to match the lightness of the PCBk test area and the lightness of the K reference area results in a large difference in the spatial frequency. Attempting to match the spatial frequencies of these two areas causes a large difference in the lightness. In either case, the patches are not appropriate for the adjacency comparison method.
The present invention provides a test pattern printing method, an information processing apparatus and a printing apparatus which are capable of printing test patterns that allow visual detection of a color deviation with high precision in a process of correcting the color deviation resulting from variations of output characteristics among printing apparatuses.
In the first aspect of the present invention, there is provided a test pattern printing method of printing a test pattern used in correction processing, the correction processing making an output characteristic for each of a plurality of colorants a predetermined one to correct a color deviation, the method comprising the step of:
printing a plurality of patches, each of which has a test area and a reference area placed adjacent to each other, the test area being printed with a mixture of the plurality of colorants and serving as an indicator for indicating a degree of the color deviation and the reference area being printed with an achromatic color and serving as a reference to be compared with the test area, the plurality of patches being formed with dots of the plurality of colorants to be printed at different mixing ratios of the plurality of colorants, respectively;
wherein the test area and the reference area are printed so that their spatial frequencies of the dots are close to each other.
In the second aspect of the present invention, there is provided an information processing apparatus for performing a process for printing a test pattern used in correction processing, the correction processing making an output characteristic for each of a plurality of colorants a predetermined one to correct a color deviation in printing by a printing apparatus, the apparatus comprising:
means for supplying printing data to cause the printing apparatus to print a plurality of patches, each of which has a test area and a reference area placed adjacent to each other, the test area being printed with a mixture of the plurality of colorants and serving as an indicator for indicating a degree of the color deviation and the reference area being printed with an achromatic color and serving as a reference to be compared with the test area, the plurality of patches being formed with dots of the plurality of colorants to be printed at different mixing ratios of the plurality of colorants, respectively;
wherein the test area and the reference area are printed so that their spatial frequencies of the dots are close to each other.
In the third aspect of the present invention, there is provided a printing apparatus capable of printing a test pattern used in correction processing, the correction processing making an output characteristic for each of a plurality of colorants a predetermined one to correct a color deviation, the apparatus comprising:
means for printing a plurality of patches, each of which has a test area and a reference area placed adjacent to each other, the test area being printed with a mixture of the plurality of colorants and serving as an indicator for indicating a degree of the color deviation and the reference area being printed with an achromatic color and serving as a reference to be compared with the test area, the plurality of patches being formed with dots of the plurality of colorants to be printed at different mixing ratios of the plurality of colorants, respectively;
wherein the test area and the reference area are printed so that their spatial frequencies of the dots are close to each other.
According to the above configuration, the test pattern, which is used to be checked for making the output characteristic for each of a plurality of colorants a predetermined one so as to correct color deviations, is printed. The test pattern comprises a plurality of patches, each of which has a test area and a reference area close together. The test area is printed with a mixture of a plurality of colorants and serves as an indicator for indicating a degree of the color deviation, and the reference area is printed with an achromatic color and serves as a reference to be compared with the test area. Among the plurality of patches, the mixing ratio of the colorants is changed from one patch to another. The spatial frequencies of the dots in the test area and in the reference area are set close to each other. Therefore, the visual perception characteristics for both areas, when the user views the patch, can be made almost equal. This, in turn, allows the user to perceive the actual colors of the two areas more precisely, making it possible to compare with good accuracy the color of the test area with that of the reference area. As a result, the user can easily discern a patch having the color of the test area closest to the color of the reference area.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.